The plussigns are anagrams bdavies .
For example ...1+17 on the down clues
Rebound and bounder .
Return and swine .
Took a while for the penny to drop for me too .)
Thanks Lie-in King! Never heard of POE-BIRD but Mamyalynne defines and it makes perfect sense. So HURRY UP is correct then for 17ac? Again, can't work out why.